From what I understand, the plastic thingies that come in the turkey are the least reliable. I've had them pop up both over and underdone, and now I just pull them out before I cook the turkey. I'd go with the Thermapen.

I go by the looseness of the leg and what color are the juices if you stick a small pointed knife into the thigh near the breast. If you know the temp of your hot water, test your thermometers under it running. Or boil some water and then insert the thermometer in it to test it. You know water boils at
212ºF.

__________________
Illegitimi non carborundum!
I don't want my last words to be, "I wish I had spent more time doing housework"

Not today, but someday use a baked potato and put both probes in close to each other and see if they can sort out an agreement betweenst them.

We baked the turkey at an even 325 oven. The bag said xx minutes per pound, and I used a probe thermometer to double check. Since your WSM, at least early on, the cooking temp was nearing an optimal temp, not sure how long it took to get there, or if it held steady, I think if it's 20 lb bird and the wrapper either said 15 or 20 min/ lb, ( I forget which), I'd go with 18 minutes /lb for or about 6 hours total cooking time. I checked the temp on our bird every time I basted it the last hour and it was done 15 minutes by probe earlier than the clocking time. Now, I would have had to factor in some oven heat loss every time I opened the oven door to baste, and not knowing how much each time , etc, that kind of math would be too hard. Anyways, going by time used to be what cookbooks said before we all got meat thermometers. (But then, some people's birds weren't as juicy as grandma's.) I think I'm digging a hole rather than offering anything useful. I better shut up now.

I guess when you determine it's done, put it on a tray and cover it with a foil tent to rest 20 minutes. The temp should continue to rise a little as it rests.

I go by the looseness of the leg and what color are the juices if you stick a small pointed knife into the thigh near the breast. If you know the temp of your hot water, test your thermometers under it running. Or boil some water and then insert the thermometer in it to test it. You know water boils at
212ºF.

Only at sea level. In Denver it was 206°F. In between, it's anyone's guess.

So far, my Thermapen has been the one I trust, although my probe (from the Thermapen site) has been very good. A lot can depend on just exactly where the bird is impaled at. A half inch can make a big difference.

I use a leave in probe thermometer and check it with an instant read. I always check several areas to make sure it's all cooked to the minimum.

After this it seems like a good idea to have two sources for reading temps... that you trust.

Whiska, it's a dual probe and they both read the same temps. I trust that more than my Thermapen, as that has been turning itself off and on lately. In other words blinking. I put new batteries in it this past week and apparently that wasn't it. Time for a call to the manufacturer.

RP, I noticed that just 1/2" made a big difference. This bird varied up to 15 degrees using the same source to measure in different areas, the Thermapen.

At any rate, I really need to get another thermometer. I think it's a good idea to have two on hand. Just like digital scales aren't the be all end all. It's good to have a regular balance scale to check against every now and then.